With a little time studying the manual and spending a little time on the boat checking everything out, drains, plugs, intakes, hoses, pipes, ect... You will find you really don't need to hire a FW mechanic. You know that at the dealership, its not there top paid mechanic doing oil changes/winterizing, same at car dealer, top mechanic does techincal jobs. So the likelyhood of someone at dealership missing something is higher than the boat owner that takes pride in doing job right himself. MHO, of couse.
Go to any RV supply store and buy the water heater bypass kit along with 3way valve antifreeze winterizing water system kit, (to install between fresh water tank and water pump, flip valve to draw from antifreeze jug vs water tank) this will have water pump do all the work for you, turn pump on and open each faucet til the pink stuff flows out, of course you have by passed the drained water heater.
For the A/C, I pull the intake hose, add extetion hose into antifreeze jug, turn system on til pink is discharged. For the toilet, again pull intake supply hose, install hose to go into antifreeze jug, cycle toilet pump couple times, reinstall raw water supply hose, then dump anti freeze down shower drain to fill sump, continue til pink is pumped overboard.
Here is a photo of camco waterheater bypass

If anyone wants/needs better photos I would be glad to go get some, after this mornings snow melts! Would need a day or so.
One other thought, it boils down to peace of mind. If you feel better about having a "marine" machanic do the job vs yourself.
My peace of mind is me KNOWING what was done, not hoping the mechanic didn't forget something.